Inkjet image forming methods have advantages over other image forming methods such that the process is simple; full color images can be produced relatively easily; and high resolution images can be produced even when using an inkjet recording apparatus having a simple structure.
Dye-based inks, in which a water soluble dye is dissolved in a medium such as water and mixture solvents of water and an organic solvent, have been typically used as inkjet inks. Such dye-based inks have an advantage such that the colors of images recorded by the inks are clear, but have a drawback such that recorded images have poor light resistance. In contrast, pigment-based inks, in which a pigment such as carbon black and organic pigments is dispersed in a medium, have better light resistance than dye-based inks, and therefore pigment-based inks have been investigated with enthusiasm. However, pigment-based inks have a drawback such that a nozzle clogging problem in that an inkjet nozzle is clogged with ink is caused relatively easily compared to dye-based inks.
Pigment-based inks are typically prepared by a method including preparing a preliminary dispersion in which a pigment (colorant) and a dispersant are preliminarily dispersed in an aqueous medium such as water and alcohols; subjecting the preliminary dispersion to a dispersing treatment using a dispersing device using a media such as sand mills to prepare a dispersion in which the pigment particles have a predetermined particle diameter; and then diluting the dispersion so as to have a predetermined concentration to prepare an ink.
Aqueous pigment-based inks typically include a surfactant or a water-soluble resin to disperse a pigment, which is typically hydrophobic. However, such aqueous pigment-based inks have poor resistance to light and water. Therefore, in order to improve image quality, a particulate resin capable of forming a film is typically included in such aqueous pigment-based inks. However, it is difficult for such an aqueous pigment-based ink that the plural ink components thereof are finely dispersed in the ink for a long period of time. When it is attempted to stably disperse such fine particles in the ink by using a large amount of dispersant such as surfactants, other problems such that air bubbles are formed in an ink tank and a recording head; and image quality deteriorates tend to occur. In addition, in attempting to improve the dispersibility of such fine particles, techniques such that a pigment having a hydrophilic group on the surface thereof is used; or a resin having a hydrophilic group is used for such inks have been proposed. However, a problem in that such a hydrophilic group causes agglomeration of a resin, thereby deteriorating the preservability of the ink is often caused.
In attempting to solve the problems, various techniques have been proposed.
For example, a recording medium for use in pigment-based inkjet recording methods is proposed, which includes a substrate and one or more ink receiving layers located on the substrate, wherein the outermost layer of the ink receiving layers includes ultrafine inorganic particles having an aggregation particle diameter of from 150 nm to 500 nm while including fine particles having an aggregation particle diameter of from 1 μm to 2 μm in a weight ratio of from 5/95 to 50/50, and the glossiness ratio of an image portion of the recording medium to a white portion (non-image portion) of the recording medium is specified. It is described therein that inkjet ink images having good visual property can be produced on the recording medium. In addition, an inkjet recording material is disclosed which includes a colorless or pale-colored solution including an electrolyte, a water-soluble organic solvent and water, and an ink including a pigment self-dispersible in water, an anionic compound, a water-soluble organic solvent, a surfactant and water, wherein the number of particles having a particle diameter of not less than 0.5 μm in 1 liter of the inkjet recording material is not less than 1×1011 and the number of particles having a particle diameter of not less than 5 nm in 1 liter of the inkjet recording material is not less than 1×109. It is described therein that by performing a heat inkjet recording method using the inkjet recording material, images having a good combination of image density, fixability, preservability and ejection stability can be produced. Further, an aqueous inkjet ink composition is disclosed which includes a pigment wherein at least 70% of the pigment particles have an average particle diameter of less than 0.1 μm with the remaining particles in the ink composition having a particle diameter of 0.1 μm or less, water, an aldehydenaphthalenesulfonate dispersant and/or a sulfone solvent. It is described therein that the ink composition has good stability and can produce high density images.
However, the dispersion stability of the inkjet inks mentioned above is not sufficient. Specifically, when pigments, which are easily damaged by strong impact in a dispersing process, are used for the inks, the resultant inks have poor dispersion stability, resulting in deterioration of the ejection stability of the inks (i.e., stability of ink ejected from an inkjet recording head).
In attempting to improve the ejection stability, an inkjet ink is disclosed which includes a wetting agent, a surfactant, a penetrant and water and in which the water evaporation rate, the increasing rate of viscosity of the ink due to evaporation of water, and the particle diameter of the colorant in the ink are specified. In addition, an inkjet ink composition is disclosed which includes an agglomerating agent, a water-soluble solvent, a surfactant and water, wherein the product of the conductivity and viscosity of the ink is specified, and the surface tension of the water-soluble solvent is also specified. Further, an inkjet ink is disclosed in which a pigment covered with a film forming resin having a carboxyl group is dispersed, wherein the resin-covered pigment has a quaternized carboxyl group on the surface thereof while having a volume average particle diameter of not greater than 0.5 μm.
However, in order to prepare an inkjet ink which has good ejection stability and can produce high quality images having a good combination of image density, visual property, feeling and glossiness, it is necessary to improve these inks.
Further, a pigment dispersion for inkjet inks is disclosed which includes a carbon black, a dispersant and water, wherein the number of coarse particles included in 5 μl of the dispersion is not greater than 1,000,000. It is described therein that an ink using the pigment dispersion can produce high quality images having a good combination of image density evenness, fixability, and preservability.
It is necessary for the ink to further improve the ejection stability in continuous inkjet recording.
For these reasons, the inventors recognized that there is a need for an inkjet ink which has a good combination of ejection stability, preservability and antifoaming property and which can produce high quality images having a good combination of image density, visual property, feeling and glossiness.